Temperature and frequency compensated array beam steering unit

ABSTRACT

Beam pointing errors and sidelobes resulting from temperature variations onhe aperture and power divider of a series phased array antenna operating at a plurality of selected operating channel frequencies and having uniformly spaced elements are compensated for by placing a temperature sensor on both the aperture and power divider and converting the respective temperature outputs to digital signals which are fed to a digital beam steering unit. A pair of programmed memories are included in the beam steering unit which respond to an address or pointer corresponding to the digitized temperature values and the selected operating frequency to read out stored digitized beam steering phase gradients and feed phase corrections which are combined and sequentially applied at regular intervals to symmetrically located phase shifter pairs. A uniformly compensated beam is thereafter radiated at a predetermined phase angle.

STATEMENT OF GOVERNMENT RIGHTS

The Government has rights in this invention pursuant to Contract No.DAAK80-80-C-0035 ordered by the Department of the Army.

BACKGROUND OF THE INVENTION

This invention relates generally to phased array antennas and moreparticularly to the compensation of beam pointing errors resulting fromfrequency and temperature changes.

In airborne and ground based phased array antenna systems driven by aseries-fed power divider and operating over a selected range offrequencies, beam pointing errors and undesired antenna sidelobes areknown to be generated as a function of the expansion and contraction ofthe aperture as a function of temperature and more particularly over therange from -50° C. to +70° C. when operating at a predeterminedfrequency. Phased errors resulting from expansion and contraction of themanifold of the power divider are also known to exist as a result oftemperature changes. Heretofore, compensation for these errors wasaccomplished by constructing the antenna components from relativelyexpensive material having a low temperature coefficient of expansion, atypical example being invar, an alloy of iron and nickel. Thismechanical approach has been found to be relatively costly and, at most,an approximation.

Accordingly, it is an object of the present invention to provide animprovement in phased array antenna systems.

It is a further object of the invention to provide an improvement in thecompensation for pointing angle errors introduced by dimensional changesof the antenna beam forming structure.

It is yet another object of the invention to provide electricalcompensation for pointing errors and sidelobes due to temperature in aphased array antenna.

And it is still a further object of the invention to electronicallyshift the antenna beam forming angle to compensate for pointing angleerrors and sidelobes introduced by dimensional changes of the phaseantenna beam forming structure due to temperature changes when operatedover a relatively large frequency band.

SUMMARY

Briefly, the foregoing and other objects of the invention areaccomplished by a method and apparatus implemented in an array beamsteering unit whereby beam pointing errors and sidelobes resulting fromtemperature variations on the aperture and power divider of a series-fedphased array antenna operating over a large range of frequencies arecompensated for electronically. A temperature sensor is placed on boththe aperture and power divider of the array which includes uniformlyspaced elements. Temperature signals corresponding to the temperature ofthe aperture and power divider are converted to digital signals whichare applied along with a digital signal corresponding to the selectedoperating frequency to digital circuit means which generate addresspointers for respective non-volatile programmed memories which read outdigitized beam steering phase gradients and feed phase corrections,respectively, which are summed and applied at regular intervals tosymmetrically located phase shifters coupled to respective elements ofthe array in accordance with a predefined sequence.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present invention is defined in the claims annexed to andforming a part of the specification, a better understanding can be hadby reference to the following description when taken in conjunction withthe accompanying drawings in which:

FIG. 1 is a block diagram generally illustrative of the subjectinvention;

FIG. 2 is a diagram further illustrative of the phased array antennashown in FIG. 1;

FIG. 3 is a block diagram further illustrative of the beam steering unitshown in FIG. 1; and

FIGS. 4A and 4B are diagrammatic illustrations of the programmable readonly memories utilized in the beam steering unit shown in FIG. 3 forproviding respective outputs for determining coarse phase gradients andfeed phase corrections.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, reference numeral 10 of FIG. 1 denotes aphased array antenna aperture including a plurality of uniformly spacedradiating elements 12 which generate a composite beam 14. The beam 14defines a phase gradient φ as shown in FIG. 2 which is perpendicular tothe direction of radiation. A beam pointing angle (θ) offset fromantenna boresight 15 is defined by the equation: ##EQU1## where φ is theincremental phase between elements, λ is the wavelength in air and d isthe physical spacing between elements. This is furthermore showndiagrammatically in FIG. 2. This figure additionally discloses aplurality of phase shifters 16 which couple RF energy to respectiveradiating elements 12 as shown in FIG. 1. The phase shifters are furtherdivided into a left and right side set 18 and 20. The two sets of phaseshifters 18 and 20 are coupled to and receive RF energy from a seriescenter fed power divider 22. The power divider 22 comprises adielectrically loaded stripline power divider and is fed RF energy froma power amplifier 24 coupled to the output of an RF signal source 26which is operable at a selected one of a plurality of operatingfrequencies f consisting of, for example, 200 discrete channelfrequencies within a portion of the C band of the electromagneticspectrum.

The steering angle θ of composite beam is controlled by a beam steeringunit 28 which generates left and right fine phase command signals, aswill be subsequently explained, which are coupled to the left and rightside phase shifters 18 and 20 at regular intervals in a predefinedsequence of individual phase shifters.

The phase gradient φ for any steering angle θ can be defined by solvingequation (1) for φ, or ##EQU2##

If it is assumed that φ and λ are invariant versus temperature, then θis dependent on variations of element spacing d (FIG. 2) withtemperature and d can be expressed as:

    d=d[1+e(T-20)]                                             (3)

where d is the nominal spacing at the standard temperature (typically20° C.), e is the coefficient of expansion in inch/inch/°C., and T isthe temperature expressed in degrees centigrade. It is known, however,that phase gradient φ does change as a function of temperature. For aseries center-fed power divider, a V-shaped gradient 14' results acrossthe array as shown in FIG. 1. By substituting equation (3) into equation(1) and (2), and referencing the result to φ as computed at 20° C., anexpression for the error Δθ as a function of temperature results whichcan be expressed as: ##EQU3## where θ₂₀ is the beam pointing angle at20° C.

This now leads to a consideration of the inventive concept of thesubject invention wherein it is desired to compensate for any change inphase gradient with respect to temperature caused by both the antennaaperture 10 and the power divider 22 for a particular operatingfrequency. This involves adding a separate temperature sensor 20 and 32,respectively, to the physical structures of the antenna aperture 10 andthe power divider 22. Both temperature sensors are operable to generatean analog electrical output signal which is fed to respective analog todigital (A/D) converters 34 and 36 which generate prescaled andquantized digital words which are used as inputs to the beam steeringunit (BSU) 28. As shown in FIG. 3, the BSU 28 includes digital means forgenerating phase shifter drive signals which are applied one at a timeto each of the phase shifters 16 in a predetermined sequence.

The compensation scheme involves the following consideration. If oneexamines equation (2) and rewrites it with d as a function oftemperature normalized to 20° C. and replaces λ by c/f, where c is thevelocity of propagation and f is the operating frequency, the expressionfor the phase gradient φ can be expressed as: ##EQU4## Expressed in thisform, all terms are constant except f and T. Compensation accordinglycomprises the selection of a proper set of beam steering gradients for aparticular operating frequency or channel and sensed temperature andinvolves generating an address or pointer to a programmed digital memorywhich will now be explained.

Beam steering is accomplished by a method which is known as theCOARSE/FINE scanning technique. With this technique, the steering phasefor each antenna element 12 is calculated at discrete steps known as thecoarse scan step. These phases are then applied at regular intervals tosymmetrically located phase shifter pairs 16 as shown in FIGS. 1 and 2according to a predefined sequence. Each pair of phase shifters in turncauses the beam 14 to move a fraction of the coarse scan step, and isknown as the fine scan step.

Referring now to FIG. 3, disclosed there are the details of the beamsteering unit 28. It comprises a digital compiler wherein analogvoltages representative of each temperature i.e. of the antenna aperture10 and power divider are preconditioned, and digitized through analog todigital converters 34 and 36 which generate 6-bit digital outputs. Thesedigital temperature values are fed via data buses 38 and 40 to digitaldata latches 42 and 44 where the binary values are temporarily storedand updated periodically under the control of the scan gate controlsignal applied from a source, not shown. The channel RF frequencyselected by the operator is also fed as an 8-bit binary word to a datalatch 46. Both the quantized temperature and channel frequency data arethus both latched at the start of an angular guidance scan interval.

Disregarding feed phase correction for the present, the steering phasegradient φ for a beam pointing angle θ degrees from boresight 15, asshown in FIG. 2, can be expressed as equation (5) above. By lumpingconstants, equation (5) can be rewritten as:

    φ=K sin θf[1-e(T-20° C.)]                 (6)

where θ is steering angle, f is operating frequency and T is temperaturein degrees centigrade. Therefore,

    φ=F(f, T)                                              (7)

The determination of the coarse phase gradient can thus be calculated asa function of both frequency and temperature. This can be realized inview of the foregoing considerations. An uncompensated increase infrequency over some nominal values produces a pointing error in the samedirection as that caused by an increase in the antenna aperturetemperature over a given ambient temperature value. By inverting theanalog temperature and prescaling the amplitude prior to digitizing, thequantized temperature may be directly added to the binary channeladdress forming an effective address or pointer that locates the coarsephase gradient value that satisfies the frequency and temperatureparameters by effecting a shift in frequency or λ. Accordingly, the beamsteering unit 28 includes a first PROM 48 which includes stored valuesof the term 2πd/λ which are read out in response to an 8-bit addresspointer generated by an address computational logic block 50 and appliedthereto via the digital data bus 52. The logic block 50 basicallycomprises a binary adder which sums the binary values of the temperatureand frequency temporarily stored in the latches 42 and 46. The followingTable A is illustrative of three resulting addresses for three differentchannel frequencies and three different temperatures, although it ispossible for different frequencies and temperatures to provide the samecoarse phase gradient address as shown in Table B.

                  TABLE A                                                         ______________________________________                                        FREQ.   +            TEMP. =   COARSE PHASE                                   Channel #                                                                             Dig. Addr.                                                                              °C.                                                                           Dig. Value                                                                            Gradient Addr.                               ______________________________________                                        0       00000000   0     011100  00011100                                     4       00000100  20     010100  00110000                                     20      00010100  40     001100  00100000                                     ______________________________________                                    

                  TABLE B                                                         ______________________________________                                        FREQ.   +            TEMP. =   COARSE PHASE                                   Channel #                                                                             Dig. Addr.                                                                              °C.                                                                           Dig. Value                                                                            Gradient Addr.                               ______________________________________                                        0       00000000  +20    010100  00010100                                     4       00000100  +30    010000  00010100                                     20      00010100  +70    000000  00010100                                     ______________________________________                                    

The PROM 48 is programmed to satisfy the function φ=F(f,T) for all ofthe required combinations of the frequency and temperature variables.Accordingly, a digital word corresponding to the value 2πd/λ isoutputted for a particular memory address which is a function oftemperature (-50° C. to +70° C.) and frequency (200 channels).Furthermore, as shown in FIG. 4A, the address pointer has a loweraddress number for increasing temperature, and decreasing frequency. A16-bit binary output from the PROM 48 is fed via a 16-bit data bus 54 toa digital multiplier 56 which receives a 16-bit word from data bus 58from a second PROM 60 which has a set of sin θ values stored therein fora plurality of beam steering command angles θ and which is fed to anaddress counter 62 under the control of a coarse clock input from asource, not shown. The multiplier 56 provides a 16-bit (2πd/λ) sin θoutput signal in accordance with equation (2) on data bus 64 whichcomprises a compensated coarse phase gradient control signal for thephase shifters 16 shown in FIG. 2. In absence of any feed phasecorrection, the coarse phase gradient signal on data bus 64 is appliedto a digital multiplier 66 where an element position number ismultiplied therewith from an input from an antenna element positioncounter 68. The output of the multiplier is rounded to a 4-bit digitalword (MSB=180°) corresponding to the fine phase drive signals applied tothe appropriate phase shifter 16 in accordance with the sequenceestablished. Thus each phase shifter pair receives its appropriate finephase drive signal in turn with the fine phase received by a particularphase shifter being the 2's complement of the fine phase received by itssymmetrically located mate.

In order to also provide for feed phase correction, the digital 6-bittemperature value of the power divider 22 temporarily stored in thelatch 44 is fed to an address computational logic block 70 along withthe 8-bit binary address of the operator selected channel frequencywhich is temporarily stored in the latch 46. The two binary valuesstored in the latches 44 and 46 are summed together in logic block 70 inthe same fashion as shown in Table A to generate a feed phase correctionaddress which appears as an 8-bit signal on data bus 72 for addressing athird PROM 74 which has a set of stored values of the term 2πd/λ_(g)where λ_(g) is the wavelength in the series-fed power divider and whichcan be expressed by the equation: ##EQU5## where λ₀ is free spacewavelength and E is the dielectric constant of the stripline dielectric.

The PROM 74 is similar to that of PROM 48 with the exception that alower address is generated for decreasing frequency. A 16-bit digitalword is fed out on the digital bus 76 where both the coarse phasegradient and the feed phase correction values are summed together in abinary adder 78. The output of the adder 78 is fed to the elementposition multiplier 66 whereupon the combined value of the coarse phasegradient and the feed phase correction is multiplied by the elementposition number to provide the respective fine phase drive signal forthe appropriate phase shifter 16.

Thus what has been shown is a beam steering unit 28 which fetchesdigitized steering phase gradients and feed phase correction data from apair of non-volatile memory storage units which are used to generatefine phase drive signals for a phased array antenna that is nowcompensated for with respect to both temperature and frequency.

Having thus shown and described what is at present considered to be thepreferred embodiment of the invention, it should be noted that the samehas been made by way of illustration and not limitation. Accordingly,all modifications, alterations and changes coming within the spirit andscope of the invention are herein meant to be included.

We claim:
 1. A method of electronically compensating for beam pointingerrors and sidelobes resulting from temperature and frequency changes ona phased array antenna including a set of phase shifters for controllingthe beam radiated from a plurality of radiating elements comprising thesteps of:sensing temperature at least of the antenna aperture; selectingan operating frequency from a plurality of discrete operatingfrequencies for RF signals transmitted from the array; converting thesensed temperature of the antenna aperture to an electrical signal;generating an electrical signal indicative of said operating frequency;combining both electrical signals and generating therefrom a memoryaddress signal; addressing a memory having a set of stored values ofbeam steering phase gradients compensated for both temperature andoperating frequency; reading out a predetermined phase gradient valuefrom said memory in accordance with said memory address signal;generating a beam steering angle command signal; operating on thepredetermined value of the phase gradient read out from the memory by amathematical function of beam steering angle and generating therefrom acoarse phase gradient signal; utilizing said phase gradient signal as aphase shifter drive signal; and coupling said drive signal to said setof phase shifters for radiating a compensated beam from said array atsaid beam steering angle.
 2. The method as defined by claim 1 andadditionally including the step of multiplying the phase gradient signalby a number corresponding to element position on the array andgenerating thereby a fine phase signal and thereafter utilizing saidfine phase signal as said phase shifter drive signal.
 3. The method asdefined by claim 2 wherein said memory comprises a digital memory,wherein said step of converting said sensed temperature to an electricalsignal comprises converting the temperature signal to a digitaltemperature signal, wherein said step of generating an electrical signalindicative of the operating frequency comprises the step of generating adigital frequency signal, and wherein said step of combining bothelectrical signals comprises summing the digital temperature andfrequency signals and generating thereby a digital memory address signalor pointer for said digital memory.
 4. The method as defined by claim 3wherein said digital memory includes a set of stored values of the term2πd/λ, where d is the separation between the radiating elements and λ isinversely proportional to frequency, and wherein said function of thebeam steering angle comprises the function sin θ, where θ is the beamsteering angle.
 5. The method as defined by claim 1 wherein said phaseshifters and said radiating elements receive RF energy of a selectedoperating frequency by way of a power divider and additionally includingthe further steps of:sensing the temperature of the power divider;converting the sensed temperature of the power divider to an electricalsignal; combining the power divider temperature signal with theoperating frequency signal and generating therefrom another memoryaddress signal; addressing another memory, said another memory having aset of stored values of feed phase corrections compensated for bothtemperature and operating frequency; reading out a predetermined feedphase correction value from said another memory in accordance with saidanother memory address signal; combining said predetermined phasegradient value and said predetermined feed phase correction value into acomposite signal; and utilizing said composite signal as said phaseshifter drive signal.
 6. The method as defined by claim 5 andadditionally including the step of multiplying said composite signal bya number corresponding to a radiating element position and generatingthereby a fine phase signal;utilizing said fine phase signal as saidphase shifter drive signal; and wherein said step of coupling said drivesignal to said set of phase shifters comprises coupling said fine phasesignal to said set of phase shifters in a predetermined sequence.
 7. Themethod as defined by claim 6 wherein said converting steps compriseconverting the respective electrical signals to digital signals, saidmemories comprise digital memories respectively storing digital valuesof phase gradients and feed phase corrections, and wherein saidcombining steps include generating respective digital memory addresssignals for addressing said digital memories.
 8. The method as definedby claim 7 wherein said set of stored values of beam steering phasegradients comprises a set of digital values of the term 2πd/λ where d isthe spacing between said radiating elements, and λ is proportional tofrequency, andwherein said set of stored values of feed phasecorrections comprises a set of digital values of the term 2πd/λ_(g)where d also comprises the spacing between said radiating elements andλ_(g) is the wavelength of energy propagating in said power divider, andsaid function of the beam steering angle comprises the function sin θwhere θ is the beam steering angle.
 9. The method as defined by claim 7wherein said combining steps comprise the steps of summing the digitalsignals indicative of aperture temperature and operating frequency togenerate a first digital memory address signal and summing the digitalsignal indicative of power divider temperature and operating frequencyto generate a second digital memory address signal.
 10. Apparatus forelectronically compensating for beam pointing errors and sidelobesresulting from temperature and frequency changes on a phased arrayantenna comprising:a linear array of a plurality of equally spacedradiating elements including a mechanical aperture; a set of phaseshifters coupled to said radiating elements for controlling the phase ofRF energy radiated from respective elements and generating a compositeradiated beam having a phased radiant thereacross; a power divider forcoupling RF energy of a predetermined operational frequency to saidphase shifters from a common source; means for sensing the temperatureat least of said antenna aperture; means for converting the sensedantenna aperture temperature to an electrical temperature signal; meansfor selecting one of a plurality of operating channel frequencies of RFenergy radiated from the radiating elements; means for generating anelectrical frequency signal of the selected channel frequency; means forcombining both electrical temperature signal and the electricalfrequency signals and generating therefrom a memory address signal; amemory including a set of stored values of beam steering phase gradientscompensated for both aperture temperature and operating frequency, saidmemory address signal reading out a predetermined value of phasegradient from said digital memory; means for generating a beam steeringangle command signal; means for multiplying the predetermined value ofthe beam steering phase gradient read out from said memory by the sinefunction of the beam steering angle to generate a coarse phase gradientsignal; means for multiplying the coarse phase gradient signal by anumber corresponding to element position and generating thereby a finephase signal; and means for coupling said fine phase signal to said setof phase shifters in a predetermined sequence for radiating acompensated beam from said array at said beam steering angle.
 11. Theapparatus as defined by claim 10 wherein said memory comprises a digitalmemory including a set of addressably stored digital values of phasegradients.
 12. The apparatus as defined by claim 11 wherein saidelectrical temperature and frequency signals comprise digital signalsand wherein said means for combining both said signals for generating amemory address signal comprises an address computational logic circuitwhich is operable to add said digital signals.
 13. The apparatus asdefined by claim 12 wherein said digital memory includes a set of storedvalues of the term 2πd/λ, where d is the spacing between radiatingelements, and λ is a function of the frequency of the RF signal radiatedfrom said elements.
 14. Apparatus for electronically compensating forbeam pointing errors and sidelobes resulting from temperature andfrequency changes on a phased array antenna fed RF energy of apredetermined operating frequency selected from a plurality of operatingchannel frequencies comprising:a plurality of equally spaced radiatingelements arranged in a linear array and driven by a plurality of phaseshifters, one for each element; a power divider coupling a common sourceof RF energy of said predetermined operating frequency to each phaseshifter; means for sensing the temperature of said antenna; firstconverting means for converting the sensed temperature to a firstelectrical temperature signal; means for sensing the temperature of saidpower divider; second converting means for converting the sensedtemperature of said power divider to a second electrical temperaturesignal; means for generating an electrical signal indicative of saidoperating frequency; first signal combining means for combining saidfirst electrical temperature signal and said electrical signalindicative of the operating frequency and generating a first memoryaddress signal; second signal combining means for combining said secondelectrical temperature signal and said electrical signal indicative ofthe operating frequency and generating second memory address signals; afirst memory responsive to said first memory address signal and having aset of retrievably stored values of beam steering phase gradientscompensated for both temperature and operating frequency and providing apredetermined phase gradient value output signal in response to aparticular first memory address signal; a second memory responsive tosaid second memory address signal and having a set of retrievably storedvalues of feed phase corrections compensated for both temperature andoperating frequency and providing a predetermined feed phase correctionvalue output signal in response to a particular second memory addresssignal; means for generating a beam steering angle command signal; meansfor operating on the predetermined phase gradient value from said firstmemory by a function of the beam steering angle to generate a coarsephase gradient signal; means for combining the coarse phase gradientsignal and the predetermined phase correction signal provided by saidsecond memory to provide a composite drive signal; said composite signalbeing utilized to drive said phase shifters for radiating a compensatedbeam from said array at said beam steering angle.
 15. The apparatus asdefined by claim 14 and additionally including means for multiplyingsaid composite signal by a number corresponding to each element positionof said array and generating thereby a fine phase signal, said finephase signal being coupled to each phase shifter in a predeterminedsequence according to element position.
 16. The apparatus as defined byclaim 15 wherein said array comprises a series-fed array and whereinsaid phase shifters comprise a first set of phase shifters for one halfof said radiating elements and a second set of phase shifters for theother half of the radiating elements and wherein said fine phase signalis applied to the respective phase shifter of said first and second setsof phase shifters in the same sequence.
 17. The apparatus as defined byclaim 15 wherein said first and second memory address signals comprisedigital address signal pointers, said first and second memories beingcomprised of digital memories and being responsive to said digitaladdress signal pointers to provide digital output signals of said phasegradients and feed phase corrections.
 18. The apparatus as defined byclaim 17 wherein said function of the beam steering angle comprises thesine function.
 19. The apparatus as defined by claim 18 wherein saidfirst digital memory includes a set of stored values of the term 2πd/λ,where d is a measure of the physical separation between radiatingelements, and λ is inversely proportional to frequency, and wherein saidsecond digital memory includes a set of stored values of the term2πd/λ_(g) where d is also said separation between radiating elements andλ_(g) is inversely proportional to the frequency of RF energypropagating in said power divider.
 20. The apparatus as defined by claim17 and additionally including a first temporary memory coupled from saidfirst converting means to said first combining means for temporarilystoring said first electrical temperature signal, a second temporarymemory coupled from said means generating said operating frequencysignal to said first and second combining means for temporarily storingsaid operating frequency signal, and a third temporary memory coupledfrom said second converting means to said second combining means fortemporarily storing said second electrical temperature signal, saidtemporary memories being operable to periodically couple therespectively stored electrical signals of temperature and operatingfrequency to said first and second combining means.
 21. The apparatus asdefined by claim 20 wherein said first and second combining means arecomprised of digital address computational logic means.